The most common reason is Youtube is still processing (not uploading but processing) your video.After uploading it can take from a couple of hours to a couple of days for Youtube to fully process your video.

Another reason is your internet speed is to weak when viewing it. Youtube will show you the best resolution it thinks is best for viewing dependent on your internet speed.

Lastly, Your video has to be 1920 x 1080 to start with.

In the editor your project needs to set to be 1920 x 1080 and rendered as 1920 x1080.